PSI - Issue 14

Tulsi Chouhan et al. / Procedia Structural Integrity 14 (2019) 883–890 Author name / Structural Integrity Procedia 00 (2018) 000–000

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If the above-mentioned conditions are fulfilled then one wave, two wave or three wave method can be used to calculate the stress, strain and strain rate under dynamic loading condition of a specimen. In this work, the authors used one wave method for the determination of properties of interest. It may be noted that all the schemes for material properties determination rely on 1D wave theory only, the difference is in how many strains (incident, reflected and transmitted) are considered for the determination of individual property. Eq. (1) to (3) will be used for the determination of Al specimen stress, strain and strain rate, respectively. The strain rate, ε̇ s (t) = ( �� � �� ) ε � ( ) (1) The average strain,  s (t)= ±( �� � �� ) ∫ ε � ( ). � � (2) The average stress,  (t) = ± � � � �  t (t) (3) 3. Results and Discussion Four different types of Al specimens were fabricated by powder metallurgy method to determine the influence of ball milling and higher rock salt percent on the compressive dynamic behavior. Significant variation in the density of all four specimen types indicates that processing affects the final product composition. Compressive high strain rate loading was performed on a 16 mm Titanium bars based SHPB set-up. For the dynamic compressive testing, the cylindrical Al test specimens were kept in between the incident and transmission bar. A suitable signal conditioner and the amplifier is used to power strain gauges and data acquisition system (Make: National Instruments, Card: NI-6115) is used to gather the voltages induced on the incident and transmission bars at a data storage rate of 2 mega samples per second. Before performing the experiments, the SHPB setup was calibrated for its correctness in accordance with the scheme suggested by Naik et al. (2008). The rate-dependent behavior of different types of Al specimens can be observed in Fig. 2. The Al specimen fabricated from powder as received attained increasing stresses as a function of the increasing rate of loading in the range of 168 MPa to 212 MPa, as the strain rate grown from 1920 /s to 3012 /s, respectively. The zones of the initial linear stress curve followed by strain hardening in the plastic zone as a function of the increasing rate of loading may be noted from Fig. 2(a). The Al specimen never underwent strain induced softening which is reported for many of the MMC composites, including Al-based MMCs (Liu et al. 2013). Towards the end of the loading cycle, the specimen stress drops suddenly indicating insignificant damage of the specimen. Within the experimental range, all the Al specimens were recovered intact. Fig. 2 (b) depicts the rate-dependent behavior of Al specimen if the specimen is made from powder ball milled for 24 hours. The purpose of milling is to reduce the powder size. As an outcome of the finer powder used for specimen preparation, the elastic curve modulus enhanced and the peak stress attained were higher compared to the previous case. Instead of continuously growing stress, a flat plateau is observed at the peak of stress, indicating property enhancement as a function of reducing powder size. The peak stresses attained by the milled powder specimen were in the range of 192 MPa to 243 MPa as a function of increasing strain rate from 1785 /s to 3025 /s, respectively. A gradually falling stress pattern at the end of the stress-strain curve indicates strain hardening followed by heat induced softening of the specimen. The phenomenon of high strain rate loading lasts for a very short period of typically 100  s to 200  s. The energy gained by the specimen due to impact is converted into heat and owing to the short span of time this heating is treated as adiabatic in nature, as the specimen will not get sufficient time to dissipate the heat. This heat induced in the specimen during high strain rate loading is responsible for thermal softening of the metallic specimen. Hence, the stress-strain curve towards the end of the loading cycle reveals gradually falling stress curve. This phenomenon of stress reduction with strain growth is attributed to adiabatic heating and microstructural damage in the literature (Zhang et al. 2009). However, the total strains attained 3.1 Dynamic compressive testing of Aluminum